Fall harvest is a good time for producers to assess the weed problems they had in soybean fields this year, and plan a good weed control program for next year. In most cases, it is no longer possible to rely strictly on glyphosate for broadleaf weed control in Roundup Ready soybeans. Before deciding what herbicide program to implement, it is necessary to identify the problem weeds in each field, and what kind of herbicide resistance issues might be present. Some good options for the most common broadleaf weed and grass problems include:
Pigweeds (including waterhemp and Palmer amaranth). Glyphosate-resistant waterhemp has exploded across eastern Kansas in recent years, and glyphosate-resistant Palmer amaranth is now becoming more common as well. For preemergence pigweed control, the Valor-based herbicides (Valor SX, Valor XLT, Fierce, Gangster, Envive, and Enlite) and Authority-based herbicides (Authority Maxx, Authority XL, Authority First, Sonic, Authority Assist, Authority MTZ, and Spartan) can all provide very good to excellent control to supplement a postemergence program. Prefix is another excellent “foundation” herbicide for residual pigweed control in soybeans and can be applied early postmergence as prior to emergence. Intrro, Dual, Outlook, and Prowl products can also provide some early-season pigweed control, but generally are not as effective as those previously mentioned products. Zidua and Anthem contain the active ingredient pyroxasulfone (also a component of Fierce). Pyroxasulfone has a similar mode of action to Intrro, Dual, and Outlook but may provide longer residual control of pigweeds.
Although some of these herbicides can be applied in the fall or early spring, those treatments generally will not persist long enough to give good pigweed control into the soybean growing season. So if pigweed is the primary target, treatments will be most effective if applied no more than two weeks prior to planting. If glyphosate-resistant pigweeds escape preemergence control, the primary postemergence tank-mix options would be Cobra, Flexstar, or Ultra Blazer. However, timing is critical for good control. These products should be applied before pigweeds exceed 4 inches tall for optimum control.
Velvetleaf. Glyphosate is not always entirely effective on velvetleaf. To assist in velvetleaf control, the Valor-based and FirstRate-based herbicides (Valor SX, Valor XLT, Fierce, Gangster, Authority First, and Sonic) are some of the most effective preplant and preemergence herbicides you can use. Postemergence tank-mixes to enhance velvetleaf control would include FirstRate, Cadet, and Resource.
Cocklebur. The most effective preplant and preemergence herbicides to aid in cocklebur control are those that contain First Rate, Classic, or Scepter. Such products would include Authority First, Sonic, Gangster, Envive, and Valor XLT. Extreme, which is a premix of glyphosate and Pursuit, can also be used as a preplant or postemergence treatment in Roundup Ready soybeans to provide residual cocklebur control. However, all of these herbicides are ALS-inhibiting herbicides, and ALS-resistant cocklebur may be present in some fields.
Marestail. Marestail is probably the most widespread glyphosate-resistant weed in Kansas. Marestail control in fields going to soybeans should begin with fall or early spring herbicide treatments that include dicamba, 2,4-D, or an ALS-inhibiting herbicide such as Canopy EX or Autumn Super. Unfortunately, ALS resistant marestail may also be present in some fields, so a tank-mix with dicamba or 2,4-D is still recommended. Dicamba has provided better marestail control than 2,4-D in K-State research the last several years. Fall treatments should be delayed until November when most of the fall-germinating marestail has emerged. With spring applications, be aware of the intervals required between application of these herbicides and planting soybeans.
A couple of relatively new options for marestail control in soybeans without a preplant waiting interval (except on coarse soils with 2% organic matter or less) are the Kixor-containing products, Sharpen and OpTill. Sharpen is Kixor alone, while OpTill is a premix of Kixor and Pursuit. Both products can be used for burndown control of marestail anytime before soybean emergence (cracking). To optimize marestail control with Sharpen and OpTill, spray before marestail gets too big, use an adequate spray volume to insure good spray coverage, and apply in combination with a methylated seed oil. The Kixor rates that can be used in soybeans will not provide very much residual control of marestail. Other residual preplant herbicides that can help with burndown and residual marestail control include Valor and FirstRate-based herbicides, such as Valor XLT, Envive, Enlite, Authority First, Sonic, or Gangster.
Marestail is best controlled before soybean planting and before the marestail begins to bolt. FirstRate or Synchrony would probably be the most effective tank-mix partner with glyphosate for postemergence marestail control in Roundup Ready soybeans. However, if ALS-resistant marestail are present, these treatments will not be very effective. Liberty is one of the better herbicides to control marestail that has started to bolt in the spring. Liberty can be used as a burndown treatment prior to emergence of any soybeans, or as a postemergence treatment in Liberty Link soybeans.
Morningglory. Glyphosate sometimes has trouble controlling morningglory. To help get better control, you can use either Authority-based or Valor-based herbicides preplant or preemergence. Liberty can also provide good morningglory control in Liberty Link soybeans.
Kochia. Kochia is a major weed problem in western areas and has often been difficult to control with glyphosate, especially as it gets bigger. In addition, glyphosate-resistant kochia is becoming a major problem in western Kansas. Since much of the kochia emerges well before soybean planting, one of the keys to managing kochia in soybeans is to control it early in the spring before soybean planting. Research by K-State the last couple of years indicates that several preemergence herbicides can help provide control of glyphosate-resistant kochia, especially the Authority-based products listed above. Early applications of dicamba can also provide effective control of kochia, but the appropriate precipitation and preplant waiting intervals need to be followed to avoid potential soybean injury and stand loss. The Kixor-containing products Sharpen and OpTill may help with kochia burndown, but the Kixor rates that can be used in soybeans will not provide very much residual control. ALS-inhibiting herbicides may or may not provide kochia control because of the occurrence of ALS-resistant kochia.
Crabgrass and small-seeded broadleaf weeds. Glyphosate usually gives good control of most grasses, but producers may want to apply a foundation herbicide to control grasses early, followed by a postemergence glyphosate application to clean up any escapes. Prefix, Fierce, Intrro, Dual II Magnum, Outlook, and Prowl H2O can all provide good early-season grass and pigweed control ahead of Roundup Ready soybeans. Of these, Prefix and Fierce generally provide the best pigweed control, and Prowl H20 the least. Several residual herbicides, such as Warrant, Outlook, and metolachlor products can be applied as a postemergence tank-mix with glyphosate to provide extended residual control of grasses and broadleaves later in the season. However, it is important to understand that these products do not have postemergence activity, so they will not control emerged glyphosate-resistant pigweeds.
Liberty Link soybeans are an alternative technology to Roundup Ready, especially in the presence of glyphosate-resistant weeds. Liberty can provide effective postemergence control on a broad spectrum of weeds, but good performance is very dependent on several application factors, such as weed size, spray coverage, and humidity. The most successful Liberty Link weed control programs will utilize a good preemergence herbicide treatment at planting, followed by a timely application of Liberty when the weeds are relatively small using a minimum spray volume of 15 gallons per acre to ensure good spray coverage.
Dallas Peterson, Weed Management Specialist
Doug Shoup, Southeast Area Crops and Soils Specialist